1. Field of the Invention
The present invention relates to a method for obtaining electronic information about a color image in a form that can be stored, transmitted, or used in electronic imaging or color reproduction.
2. Background of the Art
In the reproduction process of a color original by the halftone process, either offset, flexography, gravure, or any other printing process, each pixel element of the original is separated into amounts of colors comprising a primary set of colors, usually yellow, magenta, cyan, and black, each amount separately recorded electronically, or physically on a photosensitive medium such as photographic film in terms of density or area relative to the area of the pixel. This general process is well known and is described, for example, in The Reproduction of Color, chapters 10 and 11, by J. A. C. Yule and The Reproduction of Colour in Photography, Printing, and Television, 4th edition, chapters 25 and 28, by R. W. G. Hunt. The objective is not only that the reproduced color should perceptually match the original, but also that changes (corrections) in the original can be made to overcome defects or to alter the reproduction to a desired appearance different from the original.
Due to the requirement of making color separations, as well as historical photographic separation techniques, three filters with a bandpass generally the red, green, and blue regions of the visible spectrum are usually used to separate the original color element into amounts of yellow, magenta, and cyan. Mathematically, filters are optical integrators of their bandpass region. Typical filters might be equivalent to Wratten filter numbers 29, 47, and 61. However, no filter set exactly simulates human color vision so that reproductions will not be accurate and will, therefore, need to be corrected. Furthermore, the amount of achromatic component in a color element is usually correlated to the common amount of three filter densities, or to the density of a fourth filter which passes the visible spectrum.
If a quantitative means exists to describe the appearance of a pixel element, and the separation process and an algorithm to manipulate the separation process output can closely simulate visual appearance, then an accurate reproduction can be accomplished. Color spaces have been developed based on the trichromatic nature of human color vision as quantitative descriptions of a color. Such color spaces are, for example, CIE (Commission International de l'Eclairage) L*a*b*, L*u*v*, [C.I.E. Publication 15.2, 1986] and the 1931 CIE xyY system. Others include the Adams chromatic-valence space (Wx, Wy, Wz), Hunter Lab space, CIE (Wyszecki) U*V*W* system, MacAdam line element space, the Richter LABNHU space, and the FMC-I and FMC-II spaces. Virtually all color spaces include mathematical manipulations of the CIE tristimulus values X, Y, and Z, which supposedly represent the human trichromatic response to a perceived color. The tristimulus values are usually derived as a numerical summation at discrete wavelengths across the visible spectrum incorporating the color's reflectance or transmittance characteristics, the illuminating source's emission characteristics, and the human visual response through a response function such as the CIE standard observer color matching functions. Pyschophysically, the tristimulus values represent the continuous optical integration performed by the human visual system.
One means of overcoming some of the deficiencies of color reproduction as previously discussed is to have a gamut of known colors quantified in terms of input and output parameters of the color separation process to be used. The parameters of an original color element from the separation process can then be compared to a directory of parameter values of known colors in a memory, and the amounts of the reproduction primary colors to be used determined. Where the separation parameters do not match the directory's values close enough, an interpolation can be made if the separation parameters are within the gamut of the known colors. Such a method of color reproduction is sometimes referred to as color or hue recognition and, example, is the subject of U.S. Pat. Nos. 4,626,903, 4,623,973, 4,717,954, and 4,670,780, and the citations therein. As with the color spaces cited earlier, U.S. Pat. No. 4,623,973 also relies on an orthogonal coordinate system for the measured values ("R, G, B") of the pixel elements of a scanned color surface as well as the derived chrominance/luminance color space coordinates. U.S. Pat. No. 4,656,505 also utilizes a distinct achromatic, or neutral signal, but like other methods represents, in essence, second order masking corrections after the work of Yule, supra.